The invention relates to a method and to a conveyorized system for electrolytically processing work pieces, more specifically to a method and to a conveyorized system for processing electrical printed circuit boards and other electrical circuit carriers.
Conveyorized systems as they are utilized in the printed circuit board technique for example substantially comprise a plating tank which is filled with an electrolyte and in which anodes and cathodes are arranged. A transport device conveys the work pieces to be processed through the plant, the work pieces being held in a vertical or horizontal orientation in the case of board-shaped work pieces. The transport device may be a device using transport rolls or transport clamps. For electrolytic etching, the work pieces are anodically and the counter electrodes cathodically polarized. For electrolytic metal-plating, the work pieces are cathodically polarized; the counter electrodes are the anodes. Electrolytic contact of the work pieces may be made through contact wheels or contact clamps. DE 32 36 545 A1 discloses for example a device for electroplating in which contact wheels are provided for electrically contacting electrical printed circuit boards conveyed in a horizontal orientation and in a horizontal direction of transport through a conveyorized system. Further, DE 36 45 319 C2 discloses a plant for electrolytically processing electrical printed circuit boards in which the boards are electrically contacted by way of contact clamps which also convey them through the plant. In this case, the work pieces may, or may not, have the shape of a board. The work pieces may also be electrically contacted by contact rolls. In the case of board-shaped work pieces, contact rolls extend over the entire width of the work pieces transverse to the direction of transport. To make electrical contact, it is also known to use segmented and non-segmented contact wheels that roll off the border of the board-shaped work pieces, such as electrical printed circuit boards for example.
For easy operation by the operator staff and for reasons of construction a conveyorized system usually contains several separate counter electrodes. During etching, the counter electrodes must often be removed for stripping in order to remove the metal deposit. When soluble anodes are used in electrolytic metal-plating methods, they must often be removed for purposes of maintenance, more specifically for cleaning and metal replacement. Viewed in the direction of transport, insoluble anodes often also consist of discrete portions.
For economical reasons, the counter electrodes located opposite one side of the work pieces are in practice supplied with electric current by means of one single rectifier. The counter electrodes located on the other side of the work pieces are supplied with current through another rectifier. In the printed circuit board technique, boards and films of various sizes are electrolytically processed in such a conveyorized system. They are transported a short distance apart or closely together in a column. To prevent too high a current density at the edges of the work pieces (edge-effect: increased electric field line density at the edges of the printed circuit boards) it has become known to use technically very complicated, adjustable, electrically isolating shields.
For this purpose, WO 98 49 375 A2 for example discloses a device for electrolytically processing electrical printed circuit boards in which screening shields are provided between the plane of transportation of the electrical printed circuit boards and the counter electrodes. Each shield is configured to form at least two substantially paralleled flat parts, the one shield part being arranged opposite the plane of transportation and the other shield part opposite the counter electrodes. The shields are slidably carried substantially transverse to the conveying direction. The cost involved in using such shields is high, though.
Another disadvantage of these shields is that, although these shields permit to achieve uniform coating thickness distribution even in the lateral border regions of the electrical printed circuit boards during electrolytic metal-plating, the leading and trailing edges of the printed circuit boards extending transverse to the direction of transport cannot be protected by shields since they are conducted through the plant in a continuous manner, being electrolytically processed thereby.
At the start of the production run, there are no printed circuit boards in the electroplating plant. Due to the edge-effect, the leading edge of the first printed circuit board entering the plant is processed at too high a current density. As a result thereof, the other regions of this first printed circuit board, and possibly those of the following second printed circuit board as well, are processed at too low a current density. This leads to coating thickness distribution flaws on the surface thereof which cannot be avoided using the slidable shields described in WO 98 49 375 A2 either.
In order to avoid the edge-effect at the leading and trailing edges of the printed circuit boards conducted through the plant, a plant for electrolytically metal-plating printed circuit boards must in practice be filled with dummies first (dummies: printed circuit boards unsuited for production that are used instead of the material for production). Only then can the rectifiers for supplying current be put into operation and the production boards may follow. In this way burns at the leading edge and coating thickness flaws on the production boards are avoided. During electrolytic metal-plating, burns lead to porous to powder-like metal deposits. They are occasioned by a current density which is too high for the electrolyte used. This formation of powder is not desired in the case of dummies either since, during the metal-plating of the dummies, a metal powder thus forms which, due to the turbulent flow past the dummies as they are conducted through the plant, is detached from the surfaces thereof and is carried into the electrolyte of the working area. Later, these particles are also brought to the surface of the production boards. There, they are co-deposited, which results in a disadvantageous surface roughness. Although this roughness may be reduced using complicated electrolyte filters, it cannot be avoided altogether. Therefore, burns must completely be prevented from occurring during the electrolytic metal-plating of printed circuit boards in the fine line printed circuit technique in order to avoid the production of scrap. At the end of the production run, in the case of gaps in the column of printed circuit boards or when there is a change in product, in which the printed circuit areas and/or the current density change, the dummies must also be treated the same way as described for the start of the production run since in this case as well the border regions of the printed circuit boards extending transverse to the direction of transport would be processed at an increased current density and adjacent regions on the boards would be processed at a reduced current density if dummy boards were not used. To fill the plant with dummies is very uneconomical especially when the product to be processed is often changed not the least reason being that a sufficient number of dummies must be available at the plant. For reasons of cost, the dummies are used several times so that metal layers of increasing thickness form thereon when they are used in metal-plating plants. Therefore, the coating thickness is usually much greater than the initial coating thickness on printed circuit boards to be produced so that the electrical conductivity of the metal layer is 10 to 1000 times higher than the conductivity of the electrolytically to be processed layer on the production boards. As a result thereof, the dummies are electrolytically processed in excess to the disadvantage of the production boards. Mostly, the dummies are only scrapped when they risk to damage the plant because they have become too heavy as a result of the great coating thickness or too sharp-edged due to metal nodules for example. For the reasons mentioned, it is uneconomical and, as a result thereof undesirable, for the operator of a conveyorized system to have to work with dummies in order to avoid the disadvantageous edge-effect.
During a continuous production, the distance from one printed circuit board to the following in a column of identical printed circuit boards must be small. In the ideal case, the distance should be zero. In practice, at mean current densities (6 A/dm2 for example), distances of up to 15 mm are tolerable when the useful area on the very printed circuit boards only begins at a distance of 20 mm from the border thereof. Today's requirements to higher current densities (12 A/dm2 for example) and narrower, non-usable border areas add to the problem arising from the edge-effect. Therefore, the spacing from one printed circuit board to the other must be smaller and more precisely met.
DE 39 39 681 A1 discloses a method for controlling the run in conveyorized electroplating plants in which the spacing between electrical printed circuit boards being conveyed one behind the other through the plant are sensed either directly or by way of the position of the printed circuit boards and in which the electrical currents at the anodes are turned on or off according to the result of this sensing in such a manner that the electrical field line density is approximately the same in all of the regions of the printed circuit boards. Sensors sense the distance between the successive printed circuit boards. If there is an excessive spacing in the succession of boards, those lower and upper anodes are always turned off that are at that moment located below or above the gap in the succession of boards during the transport of the printed circuit boards through the plant. Field line concentration and the resulting increased deposits on the leading and trailing edges of the printed circuit boards are to be avoided as a result thereof. In practice, this is also the case. It will be easily understood though that, as the anode pairs are turned off one after the other, not only the edges of the printed circuit boards conducted past the turned off anodes are not electrolytically metal-plated, but the entire region of the printed circuit boards as well over a length, viewed in the direction of transport, that corresponds approximately to the anodes that have been turned off. Therefore, the areas of the printed circuit boards located behind a leading edge and in front of a trailing edge are not electrolytically metal-plated or too high current densities are generated at the front and/or rear edges. Moreover, when the anodes are turned off in this way, the current of the turned off anodes is deviated to the turned on anodes so that metal-plating is carried out at an accordingly undesired higher current density. This method still permits to avoid burns and the resulting roughness of the metal deposited. It is also used for this purpose. However, this technique cannot prevent the at least two printed circuit boards located in front of and behind a gap in the printed circuit board column from being scrap. As the printed circuit boards are becoming increasingly expensive as a result of the fine line printed circuit technique and the SBU-technique (sequential build up), this scrap is not tolerated either.
In the SBU-technique, an all-over layer of electroless copper of a thickness of e.g., 0.5 μm which is to be electrolytically metal-plated is used. As compared to the electrolytic copper layers of the current printed circuit board technique of a thickness of 5-17.5 μm, this thin layer has a high ohmic resistance. As described herein above, at least part of the plant must be filled with dummies before the printed circuit boards are allowed to enter in order to permit the electrolytic metal-plating of the production boards without scrap. As compared to the SBU layer made of electroless copper, the dummies have an approximately 1000 times higher electrical conductivity. If SBU boards are introduced into the plant after dummies or if they exit the plant in front of dummies, the electrolytic current of the anodes is not distributed proportionally with regard to the surface area onto the various neighboring boards. The electric current substantially flows onto the highly conductive dummy boards. Virtually the SBU boards are not electrolytically metal-plated. If insoluble anodes are used in a chemically etching electrolyte, more specifically when the electrolyte contains compounds of a redox couple, e.g., Fe2+/Fe3+ compounds, there is a risk that regions of the SBU boards located far from the contacts are etched, that is to say completely destroyed. At best, the second or third SBU board behind or in front of a dummy is usable under these conditions. That these expensive SBU boards have to be scrapped is not accepted either.
In order to adjust in ideal manner the electrolytic metal-plating current for a first work piece immersed into an electrolyte, Patent Abstracts of Japan to JP 61133400 A suggests an electroplating device provided with an elongated plating cell for work pieces to be processed containing plating liquid and with anode plates arraying in series and separately supplied with current. The work pieces are sunk into the plating liquid at one end, the supply current of the rectifiers being gradually increased with the speed at which the work piece is sunk into the plating liquid. Burns are thus avoided.
In order to avoid the point effect during the electrolytically processing of fine circuit traces on electrical printed circuit boards which causes a varying electrical field line density to result in locally different processing effects on circuit traces of varied width, DE 44 17 551 C2 suggests to keep constant the distance between the printed circuit boards and the anodes by using electrically isolating distance members, said distance being maximum 30 times the nominal width of the narrow circuit traces.